algid, gelid: Although at first glance, these might seem like scientific taxonomic terms — phylla or species maybe — they're really just fun, fancy, Scrabble-friendly words for cold. Algid comes from the Latin algidus, cold; gelid comes from the Latin gelidus, frost. (The word gelatin stems from the same root.) Gelid temperatures are therefore colder than algid temperatures. And you can nounify both of them as algidity and gelidity.
And it's certainly gelid here in the Midwest today, with temperatures just below 0ºF, and wind chills about 15 degrees lower than that. The worst part about such frigid temperatures, aside from snot freezing in my nostrils, is that as the gelidity goes up (or would it be down?), drivers' common sense abates. I've always said that the worst part about driving is other people on the road; that goes double for wintertime driving.
On the subject of whether algidity and gelidity (and frigidity, for that matter) goes up or down as it gets colder, consider this: French astronomer Joseph-Nicholas Delisle created a mecury thermometer in 1732 for Peter the Great. Delisle's scale was based on the contraction of mercury, not on its expansion, as today's temperature scales are. In the Delisle scale, at sea level, water boiled at 0º and froze at 150°, so the higher the number, the colder it was. If that seems odd, what many people don't know is that the original Celsius scale, credited to Anders Celsius around 1742, was inverted: water froze at 100º and boiled at 0°. Other scientists later turned the scale around.
For more about temperature scales, take a look at these two articles written by a guy I know: